U.S. patent application number 15/086475 was filed with the patent office on 2016-07-21 for illumination system of a microlithographic projection exposure apparatus.
The applicant listed for this patent is Carl Zeiss SMT GmbH. Invention is credited to Vladimir Davydenko, Markus Deguenther, Stefanie Hilt, Wolfgang Hoegele, Thomas Korb, Frank Schlesener.
Application Number | 20160209759 15/086475 |
Document ID | / |
Family ID | 49626852 |
Filed Date | 2016-07-21 |
United States Patent
Application |
20160209759 |
Kind Code |
A1 |
Deguenther; Markus ; et
al. |
July 21, 2016 |
ILLUMINATION SYSTEM OF A MICROLITHOGRAPHIC PROJECTION EXPOSURE
APPARATUS
Abstract
An illumination system of a microlithographic projection
exposure apparatus includes a pupil forming unit directing light on
a spatial light modulator that transmits or reflects impinging
light in a spatially resolved manner. An objective images a light
exit surface of the spatial light modulator on light entrance
facets of an optical integrator so that an image of an object area
on the light exit surface completely coincides with one of the
light entrance facets. The pupil forming unit and the spatial light
modulator are controlled so that the object area is completely
illuminated by the pupil forming unit and projection light
associated with a point in the object area is at least partially
and variably prevented from impinging on the one of the light
entrance facets.
Inventors: |
Deguenther; Markus; (Aalen,
DE) ; Davydenko; Vladimir; (Bad Herrenalb, DE)
; Korb; Thomas; (Schwaebisch Gmuend, DE) ;
Schlesener; Frank; (Oberkochen, DE) ; Hilt;
Stefanie; (Aalen, DE) ; Hoegele; Wolfgang;
(Aalen, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Carl Zeiss SMT GmbH |
Oberkochen |
|
DE |
|
|
Family ID: |
49626852 |
Appl. No.: |
15/086475 |
Filed: |
March 31, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2014/003049 |
Nov 13, 2014 |
|
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15086475 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70066 20130101;
G02B 26/0833 20130101; G03F 7/70425 20130101; G03F 7/70116
20130101; G03F 7/70191 20130101; G03F 7/70075 20130101; G03F
7/70058 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 22, 2013 |
EP |
13194135.3 |
Claims
1. An illumination system, comprising: an optical integrator
configured to produce a plurality of secondary light sources in a
pupil plane, the optical integrator comprising a plurality of light
entrance facets, each light entrance facet being associated with
one of the secondary light sources, images of the light entrance
facets being at least substantially superimposed in a mask plane; a
spatial light modulator having a light exit surface, the spatial
light modulator configured to transmit or to reflect projection
light in a spatially resolved manner; a pupil forming unit
configured to direct projection light onto the spatial light
modulator; an objective configured to image the light exit surface
of the spatial light modulator onto the light entrance facets of
the optical integrator so that an image of an object area on the
light exit surface completely coincides with one of the light
entrance facets; and a control unit configured to control the pupil
forming unit and the spatial light modulator so that: i) the object
area is completely illuminated by the pupil forming unit; and ii)
projection light associated with a point in the object area is at
least partially and variably prevented from impinging on the one of
the light entrance facets.
2. The illumination system of claim 1, wherein the pupil forming
unit comprises a first beam deflection array of first reflective or
transparent beam deflection elements, and each beam deflection
element is configured to illuminate a spot on the spatial light
modulator at a position that is variable by changing a deflection
angle produced by the beam deflection element.
3. The illumination system of claim 1, wherein the spatial light
modulator comprises a second beam deflection array of second
reflective or transparent beam deflection elements, and each second
beam deflection element has: i) a first state configured to direct
impinging light towards the optical integrator; and ii) a second
state configured to direct impinging light elsewhere.
4. The illumination system of claim 3, wherein the second beam
deflection array comprises a digital mirror device.
5. The illumination system of claim 3, wherein at least 10 second
beam deflection elements are arranged in the object area.
6. The illumination system of claim 3, wherein centers of adjacent
second beam deflection elements arranged in the object area are
aligned along a straight line, an image of the straight line forms
an angle .alpha. to a boundary line of the one of the light
entrance facets, .alpha. is distinct from m45.degree., and m=0, 1,
2, 3, . . . .
7. The illumination system of claim 6, wherein boundaries of the
second beam deflection elements are arranged in a first rectangular
grid, boundaries of the light entrance facets are arranged in a
second rectangular grid, and an image of the first rectangular grid
formed on the light entrance facets forms the angle .alpha. to the
second rectangular grid.
8. The illumination system of claim 3, wherein a length of the
object area along a first direction is greater than a length of the
object area along a second direction which is orthogonal to the
first direction, the objective is an anamorphotic objective having
a magnification M, and |M| is less along the first direction than
along the second direction.
9. The illumination system of claim 8, wherein the second direction
corresponds to a scan direction along which the mask moves while
the mask is illuminated by the illumination system during use of
the illumination system.
10. The illumination system of claim 3, wherein the second beam
deflection elements are arranged in an object plane of the
objective that is parallel to a plane in which the light entrance
facets are arranged, and during use of the second beam deflection
elements produce in the first state a deflection of impinging light
by an angle distinct from zero.
11. The illumination system of claim 3, wherein the second beam
deflection elements are arranged in an object plane of the
objective that is parallel to a plane in which the light entrance
facets are arranged, the objective is non-telecentric on an object
side, and the objective is telecentric on an image side.
12. The illumination system of claim 3, further comprising a
scattering plate in a light path between the optical light
modulator and the mask plane.
13. The illumination system of claim 1, wherein at least one half
of all object areas on the light exit surface of the spatial light
modulator are completely illuminated by the pupil forming unit
during use of the illumination system.
14. The illumination system of claim 2, wherein, during use of the
illumination system, the light spots produced by the first beam
deflection elements on the object area are larger than the object
area.
15. The illumination system of claim 1, wherein the object area on
the light exit surface of the optical light modulator is an active
object area configured to prevent projection light associated with
a point in the active object area from impinging on the one of the
light entrance facets, and the spatial light modulator has another
object area that is a passive object area configured to avoid
preventing projection light associated with a point in the passive
object area from impinging on the one of the light entrance
facets.
16. The illumination system of claim 15, wherein, during use of the
illumination system, the irradiance produced on the spatial light
modulator by the pupil forming unit is higher on the active object
area than on the passive object area.
17. The illumination system of claim 15, wherein the passive object
area and the active object area are arranged point-symmetrically to
each other with respect to an optical axis of the illumination
system.
18. The illumination system of claim 17, wherein the optical light
modulator comprises a plurality of active object areas and a
plurality of passive object areas, and each passive object area is
arranged point-symmetrically to one of the active object areas.
19. The illumination system of claim 1, wherein the light exit
surface of the optical light modulator comprises groups of object
areas that are separated by areas that are not imaged on the light
entrance facets, and the objective is configured to combine images
of the object areas so that the images of the object areas abut on
the optical integrator.
20. The illumination system of claim 18, wherein the objective
comprises: a first array of first optical elements, each first
optical element configured to form a magnified image of one of the
groups in an intermediate image plane; and imaging optics
configured to image the intermediate image plane on the light
entrance facets.
21. A method, comprising: producing an irradiance distribution of
projection light on a spatial light modulator that has a light exit
surface and is configured to transmit or reflect projection light
in a spatially resolved manner, the light exit surface comprising
an object area that is completely illuminated by projection light;
imaging the object area on the light exit surface on a light
entrance facet of an optical integrator so that an image of the
object area completely coincides with the light entrance facet; and
controlling the spatial light modulator so that projection light
associated with a point in the object area is at least partially
prevented from impinging on the light entrance facet.
22. A method, comprising: completely illuminating an object area on
a spatial light modulator; imaging the object area on a light
entrance facet of an optical integrator; and preventing that all
light associated with a point in the object area impinges on the
light entrance facet.
23. (canceled)
24. An illumination system, comprising: an optical integrator
configured to produce a plurality of secondary light sources in a
pupil plane, the optical integrator comprising a plurality of light
entrance facets, each light entrance facet being associated with
one of the secondary light sources; a spatial light modulator
having a light exit surface, the spatial light modulator configured
to transmit or to reflect impinging projection light in a spatially
resolved manner; a pupil forming unit configured to direct
projection light on the spatial light modulator; an objective
configured to image the light exit surface of the spatial light
modulator onto the light entrance facets of the optical integrator;
and a control unit configured to control the pupil forming unit and
the spatial light modulator.
25.-43. (canceled)
44. An illumination system, comprising: a pupil forming unit
configured to direct light on a spatial light modulator that is
configured to transmit or to reflect impinging light in a spatially
resolved manner; an objective configured to image a light exit
surface of the spatial light modulator onto light entrance facets
of an optical integrator so that an image of an object area on the
light exit surface completely coincides with one of the light
entrance facets; and a control unit configured to control the pupil
forming unit and the spatial light modulator so that: i) the object
area is completely illuminated by the pupil forming unit; and ii)
projection light associated with a point in the object area is at
least partially and variably prevented from impinging on the one of
the light entrance facets.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation of, and claims
benefit under 35 USC 120 to, international application
PCT/EP2014/003049, filed Nov. 13, 2014, which claims benefit under
35 USC 119 of European Application No. 13194135.3, filed Nov. 22,
2013. The entire disclosure of international application
PCT/EP2014/003049 and European Application No. 13194135.3 are
incorporated by reference herein.
FIELD
[0002] The disclosure generally relates to illumination systems for
illuminating a mask in microlithographic exposure apparatus, and in
particular to such systems including an optical integrator
configured to produce a plurality of secondary light sources in a
pupil plane. The disclosure also relates to a method of operating
such illumination systems.
BACKGROUND
[0003] Microlithography (also referred to as photolithography or
simply lithography) is a technology for the fabrication of
integrated circuits, liquid crystal displays and other
microstructured devices. The process of microlithography, in
conjunction with the process of etching, is used to pattern
features in thin film stacks that have been formed on a substrate,
for example a silicon wafer. At each layer of the fabrication, the
wafer is first coated with a photoresist which is a material that
is sensitive to radiation, such as deep ultraviolet (DUV) light.
Next, the wafer with the photoresist on top is exposed to
projection light in a projection exposure apparatus. The apparatus
projects a mask containing a pattern onto the photoresist so that
the latter is only exposed at certain locations which are
determined by the mask pattern. After the exposure the photoresist
is developed to produce an image corresponding to the mask pattern.
Then an etch process transfers the pattern into the thin film
stacks on the wafer. Finally, the photoresist is removed.
Repetition of this process with different masks results in a
multi-layered microstructured component. A projection exposure
apparatus typically includes a light source, an illumination system
that illuminates the mask with projection light produced by the
light source, a mask stage for aligning the mask, a projection
objective and a wafer alignment stage for aligning the wafer coated
with the photoresist. The illumination system illuminates a field
on the mask that may have the shape of a rectangular or curved
slit, for example.
[0004] In current projection exposure apparatus a distinction can
be made between two different types of apparatus. In one type each
target portion on the wafer is irradiated by exposing the entire
mask pattern onto the target portion in one go. Such an apparatus
is commonly referred to as a wafer stepper. In the other type of
apparatus, which is commonly referred to as a step-and-scan
apparatus or scanner, each target portion is irradiated by
progressively scanning the mask pattern under the projection beam
along a scan direction while synchronously moving the substrate
parallel or anti-parallel to this direction. The ratio of the
velocity of the wafer and the velocity of the mask is equal to the
magnification of the projection objective, which is usually smaller
than 1, for example 1:4.
[0005] It is to be understood that the term "mask" (or reticle) is
to be interpreted broadly as a patterning mechanism. Commonly used
masks contain opaque or reflective patterns and may be of the
binary, alternating phase-shift, attenuated phase-shift or various
hybrid mask type, for example. However, there are also active
masks, e.g. masks realized as a programmable mirror array. Also
programmable LCD arrays may be used as active masks.
[0006] As the technology for manufacturing microstructured devices
advances, there are ever increasing demands also on the
illumination system. Ideally, the illumination system illuminates
each point of the illuminated field on the mask with projection
light having a well defined spatial and angular irradiance
distribution. The term angular irradiance distribution describes
how the total light energy of a light bundle, which converges
towards a particular point in the mask plane, is distributed among
the various directions of the rays that constitute the light
bundle.
[0007] The angular irradiance distribution of the projection light
impinging on the mask is usually adapted to the kind of pattern to
be projected onto the photoresist. Often the angular irradiance
distribution depends on the size, orientation and pitch of the
features contained in the pattern. The most commonly used angular
irradiance distributions of projection light are referred to as
conventional, annular, dipole and quadrupole illumination settings.
These terms refer to the irradiance distribution in a pupil plane
of the illumination system. With an annular illumination setting,
for example, only an annular region is illuminated in the pupil
plane. Thus there is only a small range of angles present in the
angular irradiance distribution of the projection light, and all
light rays impinge obliquely with similar angles onto the mask.
[0008] Different approaches are known in the art to modify the
angular irradiance distribution of the projection light in the mask
plane so as to achieve the desired illumination setting. In the
simplest case a stop (diaphragm) including one or more apertures is
positioned in a pupil plane of the illumination system. Since
locations in a pupil plane translate into angles in a Fourier
related field plane such as the mask plane, the size, shape and
location of the aperture(s) in the pupil plane determines the
angular irradiance distributions in the mask plane. However, any
change of the illumination setting involves a replacement of the
stop. This makes it difficult to finely adjust the illumination
setting, because this would involve a very large number of stops
that have aperture(s) with slightly different sizes, shapes or
locations. Furthermore, the use of stops inevitably results in
light losses and thus in a reduced throughput of the apparatus.
[0009] Many common illumination systems therefore include
adjustable elements that make it possible, at least to a certain
extent, to continuously vary the illumination of the pupil plane.
Many illumination systems use an exchangeable diffractive optical
element to produce a desired spatial irradiance distribution in the
pupil plane. If zoom optics and a pair of axicon elements are
provided between the diffractive optical element and the pupil
plane, it is possible to adjust this spatial irradiance
distribution.
[0010] Recently it has been proposed to use mirror arrays that
illuminate the pupil plane. In EP 1 262 836 A1 the mirror array is
realized as a micro-electromechanical system (MEMS) including more
than 1000 microscopic mirrors. Each of the mirrors can be tilted in
two different planes perpendicular to each other. Thus radiation
incident on such a mirror device can be reflected into
(substantially) any desired direction of a hemisphere. A condenser
lens arranged between the mirror array and the pupil plane
translates the reflection angles produced by the mirrors into
locations in the pupil plane. This known illumination system makes
it possible to illuminate the pupil plane with a plurality of
spots, wherein each spot is associated with one particular
microscopic mirror and is freely movable across the pupil plane by
tilting this mirror.
[0011] Similar illumination systems are known from US 2006/0087634
A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO
2010/006687 A1. US 2010/0157269 A1 discloses an illumination system
in which an array of micromirrors is directly imaged on the
mask.
[0012] As mentioned further above, it is usually desired to
illuminate, at least after scan integration, all points on the mask
with the same irradiance and angular irradiance distribution. If
points on the mask are illuminated with different irradiances, this
usually results in undesired variations of the critical dimension
(CD) on wafer level. For example, in the presence of irradiance
variations the image of a uniform line on the mask on the light
sensitive may also have irradiance variations along its length.
Because of the fixed exposure threshold of the resist, such
irradiance variations directly translate into widths variations of
a structure that shall be defined by the image of the line.
[0013] If the angular irradiance distribution varies over the
illuminated field on the mask, this also has a negative impact on
the quality of the image that is produced on the light sensitive
surface. For example, if the angular irradiance distribution is not
perfectly balanced, i.e more light impinges from one side on a mask
point than from the opposite side, the conjugate image point on the
light sensitive surface will be laterally shifted if the light
sensitive surface is not perfectly arranged in the focal plane of
the projection objective.
[0014] For modifying the spatial irradiance distribution in the
illumination field U.S. Pat. No. 6,404,499 A and US 2006/0244941 A1
propose mechanical devices that include two opposing arrays of
opaque finger-like stop elements that are arranged side by side and
aligned parallel to the scan direction. Each pair of mutually
opposing stop elements can be displaced along the scan direction so
that the distance between the opposing ends of the stop elements is
varied. If this device is arranged in a field plane of the
illumination system that is imaged by an objective on the mask, it
is possible to produce a slit-shaped illumination field whose width
along the scan direction may vary along the cross-scan direction.
Since the irradiance is integrated during the scan process, the
integrated irradiance (sometimes also referred to as illumination
dose) can be finely adjusted for a plurality of cross-scan
positions in the illumination field.
[0015] Unfortunately these devices are mechanically very complex
and expensive. This is also due to the fact that these devices have
to be arranged in or very close to a field plane in which usually
the blades of a movable field stop is arranged.
[0016] Adjusting the angular irradiance distribution in a field
dependent manner is more difficult. This is mainly because the
spatial irradiance distribution is only a function of the spatial
coordinates x, y, whereas the angular irradiance distribution also
depends on the angles .alpha., .beta..
[0017] WO 2012/100791 A1 discloses an illumination system in which
a first mirror array is used to produce a desired irradiance
distribution in the pupil plane of the illumination system. In
close proximity to the pupil plane an optical integrator is
arranged that has a plurality of light entrance facets. Thus images
of the light entrance facets are superimposed on the mask. The
light spots produced by the mirror array have an area that is at
least five times smaller than the total area of the light entrance
facets. Thus it is possible to produce variable light patterns on
the light entrance facets. In this manner different angular
irradiance distributions can be produced on different portions of
the illumination field. It is thus possible, for example, to
produce an X dipole and a Y dipole illumination setting at a given
time in the illumination field.
[0018] In order to ensure that the portions with different
illumination settings are sharply delimited, it is proposed to use
a second mirror array configured as a digital mirror device (DMD).
This second mirror array is illuminated by the first mirror array
and is imaged on the light entrance facets by an objective. By
bringing larger groups of micromirrors of the second mirror array
in an "off"-state, it is possible to produce irradiance
distributions on the light entrance facets that have sharp
boundaries.
[0019] However, it turned out that it is difficult to produce so
many and so small freely movable light spots with the first mirror
array. Furthermore, this prior art illumination system is mainly
concerned with producing completely different illumination settings
at different portions in the illumination field. For that reason
the light entrance facets are usually not completely, but only
partially illuminated.
SUMMARY
[0020] The present disclosure seeks to provide an illumination
system of a microlithographic projection exposure apparatus which
is capable of adjusting both the spatial and the angular irradiance
distribution in a field dependent manner. This means that it shall
be possible to adjust the irradiance and angular irradiance
distribution at different points in the illumination field
differently.
[0021] In an aspect, the disclosure provides an illumination system
including a pupil plane, a mask plane in which a mask to be
illuminated by projection light can be arranged, and an optical
integrator. The latter is configured to produce a plurality of
secondary light sources located in the pupil plane. The optical
integrator includes a plurality of light entrance facets each being
associated with one of the secondary light sources. Images of the
light entrance facets at least substantially superimpose in the
mask plane. The illumination system further includes a spatial
light modulator that has a light exit surface and is configured to
transmit or to reflect impinging projection light in a spatially
resolved manner, a pupil forming unit that is configured to direct
projection light on the spatial light modulator, and an objective
that images the light exit surface of the spatial light modulator
onto the light entrance facets of the optical integrator so that an
image of an object area on the light exit surface completely
coincides with one of the light entrance facets. A control unit is
configured to control the pupil forming unit and the spatial light
modulator so that the object area is completely illuminated by the
pupil forming unit and projection light associated with a point in
the object area is at least partially and variably prevented from
impinging on the one of the light entrance facets.
[0022] The disclosure is based on the perception that instead of
using a spatial light modulator only for producing sharp edges of
an irradiance distribution on the light entrance facets, it may
also be used to modify the irradiance distribution if the object
area imaged on a light entrance facet is completely illuminated so
that there would be no need for sharp edges.
[0023] With the spatial light modulator controlled in the manner
described above it is possible to dispense with mechanical complex
devices that are used in prior art illumination systems to adjust
the spatial irradiance distribution along the cross-scan direction,
and simultaneously to flexibly adjust the angular irradiance
distribution at mask level in a field dependent manner. Since the
geometrical optical flux is small in front of the optical
integrator, the objective that images the light exit surface of the
spatial light modulator on the light entrance facets can be
realized with very few and preferably spherical lenses.
[0024] Since all components of the illumination system may be
purely reflective, the disclosure can principally also be used in
EUV illumination systems.
[0025] The pupil forming unit may include a diffractive optical
element for defining an irradiance distribution on the spatial
light modulator that is imaged on the light entrance facets of the
optical integrator. For fine adjustments of this irradiance
distribution zoom optics and/or a pair of axicon elements may be
arranged in the light path between the diffractive optical element
and the spatial light modulator.
[0026] A more flexible setting of the irradiance distribution on
the spatial light modulator is possible if the pupil forming unit
includes a first beam deflection array of first reflective or
transparent beam deflection elements. Each beam deflection element
is configured to illuminate a spot on the spatial light modulator
at a position that is variable by changing a deflection angle
produced by the beam deflection element.
[0027] The spatial light modulator may be of the transparent or the
reflective type and may includes an array of elements that can be
used to attenuate, completely block or deflect impinging light. For
example, the spatial light modulator may be configured as an LCD
panel including a two dimensional array of LCD cells whose optical
activity can be controlled individually by the control unit. In
modulators of the transparent type the object area is usually
illuminated from its back side.
[0028] In one embodiment the spatial light modulator includes a
second beam deflection array of second reflective or transparent
beam deflection elements. Each second beam deflection element is
capable to be in an "on"-state, in which it directs impinging light
towards the optical integrator, and in an "off"-state, in which it
directs impinging light elsewhere, for example on a light absorbing
surface. Such a second beam deflection array may be configured as a
digital mirror device which may include millions of individual
micromirrors.
[0029] Generally the larger the number of second beam deflection
elements arranged in the object area is, the better is the spatial
resolution of the field dependent adjustment of the irradiance and
angular irradiance distribution. Preferably at least 10, and even
more preferably at least 50, second beam deflection elements are
arranged in the object area.
[0030] In one embodiment center of adjacent second beam deflection
elements arranged in the object area are aligned along a straight
line. An image of the straight line forms an angle .alpha. to a
boundary line of the one of the light entrance facets, wherein a is
distinct from m45.degree. with m=0, 1, 2, 3, . . . . With such an
oblique arrangement of the second beam deflection array with
respect to the light entrance facets the distance is reduced
between cross-scan positions in the illumination field at which the
attenuation is different.
[0031] For example, the boundaries of the second beam deflection
elements may be arranged in a first rectangular grid, and
boundaries of the light entrance facets may be arranged in a second
rectangular grid. Then the image of the first rectangular grid
formed on the light entrance facets forms the angle .alpha. to the
second rectangular grid.
[0032] If the mask moves along a scan direction while it is
illuminated by the illumination system, the irradiance and angular
irradiance distribution at a point on the mask is obtained by
integrating the irradiances and angular irradiance distributions
during the scan process, i.e. while the point on the mask moves
through the illumination field. For that reason it may be
sufficient to provide only a few second beam deflection elements
along the scan direction, but a larger number of second beam
deflection elements along the cross-scan direction in order to
ensure that the field dependence of the irradiance and angular
irradiance distribution can be finely adjusted. This usually
implies that a length of the object area along the first direction
should be larger than a length of the object area along a second
direction which is orthogonal to the first direction. Then the
objective should be an anamorphotic objective having a
magnification M with |M| being smaller along the first direction
than along the second direction. The anamorphotic objective ensures
that the image of the elongated object area is not elongated, but
coincides with the (usually square) shape of the light entrance
facets.
[0033] Instead of or in addition to using an anamorphotic
objective, it is possible to use an anamorphotic condenser having a
front focal plane which coincides with the pupil plane and having a
focal length f being shorter along the first direction than along
the second direction.
[0034] Generally it is preferred if the second beam deflection
elements are arranged in an object plane of the objective that is
parallel to a plane in which the light entrance facets are
arranged. This can be achieved if the second beam deflection
elements are configured such that they produce in the "on"-state a
deflection of impinging light by an angle distinct from zero.
Additionally or alternatively the objective may be non-telecentric
on an object side and telecentric on an image side.
[0035] Generally the light spots produced by the first beam
deflection array on the object area will be larger than the object
area. However, the disclosure may also be used if the spots are
smaller than the object area.
[0036] Since gaps between second beam deflection elements are, via
the light entrance facets of the optical integrator, eventually
imaged on the illumination field, measures should be taken that
this does not compromise the uniformity of the spatial and angular
irradiance distribution in the illumination field. To this end a
scattering plate may be arranged in a light path between the
optical light modulator and the mask plane, preferably close to a
field plane. The scattering plate blurs the irradiance distribution
on the light entrance facets and thus ensures that no dark lines
occur in the illumination field.
[0037] If the object area on the light exit surface of the optical
light modulator is considered as an active object area so that
projection light associated with a point in the active object area
can be prevented from impinging on the one of the light entrance
facets, the spatial light modulator may include another object area
that is a passive object area so that projection light associated
with a point in the passive object area cannot be prevented from
impinging on the one of the light entrance facets. Such a
combination of active and passive object areas may be expedient
particularly in those cases in which the spatial resolution
produced by the optical light modulator in an active object area
shall be very high. If the optical light modulator is configured as
a digital mirror device, for example, this would involve a huge
number of micromirrors because also the number of light entrance
facets is usually large. Digital mirror devices with such a huge
number of micromirrors may not yet be easily available. For that
reason it may be expedient to assemble the optical light modulator
from smaller active object areas, for example formed by
conventional digital mirror devices, and passive object areas, for
example realized as plane mirrors, in between. Such an arrangement
may be useful because often it is not necessary to modify the
irradiance distribution on every light entrance facet, but only on
some of them.
[0038] In order maintain the pole balance, the irradiance produced
on the spatial light modulator by the pupil forming unit may be
higher on the active object area than on the passive object area.
This higher irradiance compensates light losses that are caused by
preventing light from reaching the light entrance facets.
[0039] Preferably the passive object areas and the active object
areas are arranged point-symmetrically to each other with respect
to an optical axis of the illumination system. This ensures that
the energetic balance (telecentricity) of the light bundles
converging to points on the mask can always be adjusted. Then
roughly one half of the total area of the light exit surface may be
covered by active object areas and the other half by passive object
areas.
[0040] If the light exit surface of the optical light modulator
includes groups of object areas that are separated by areas that
are not imaged on the light entrance facets, the objective may be
configured to combine images of the active object areas so that the
images of the object areas abut on the light entrance facets.
[0041] In particular the objective may include a first array of
first optical elements, wherein each first optical element forms a
magnified image of one of the object areas in an intermediate image
plane, and imaging optics that image the intermediate image plane
on the light entrance facets.
[0042] Subject of the disclosure is also a method of operating an
illumination system of a microlithographic projection exposure
apparatus, including the following steps: [0043] a) producing an
irradiance distribution of projection light on a spatial light
modulator that has a light exit surface and is configured to
transmit or to reflect impinging projection light in a spatially
resolved manner, wherein the light exit surface includes an object
area that is completely illuminated by projection light; [0044] b)
imaging the object area on the light exit surface on a light
entrance facet of an optical integrator so that an image of the
object area completely coincides with the light entrance facet;
[0045] c) controlling the spatial light modulator so that
projection light associated with a point in the object area (110)
is at least partially prevented from impinging on the light
entrance facet.
[0046] Subject of the disclosure is also another method of
operating an illumination system of a microlithographic projection
exposure apparatus, including the following steps: [0047] a)
completely illuminating an object area (110) on a spatial light
modulator (52); [0048] b) imaging the object area (110) on a light
entrance facet (75) of an optical integrator (60); [0049] c)
preventing that all light associated with a point in the object
area impinges on the light entrance facet.
DEFINITIONS
[0050] The term "light" is used herein to denote any
electromagnetic radiation, in particular visible light, UV, DUV,
VUV and EUV light and X-rays.
[0051] The term "light ray" is used herein to denote light whose
path of propagation can be described by a line.
[0052] The term "light bundle" is used herein to denote a plurality
of light rays that have a common origin in a field plane.
[0053] The term "light beam" is used herein to denote all light
that passes through a particular lens or another optical
element.
[0054] The term "position" is used herein to denote the location of
a reference point of a body in the three-dimensional space. The
position is usually indicated by a set of three Cartesian
coordinates. The orientation and the position therefore fully
describe the placement of a body in the three-dimensional
space.
[0055] The term "surface" is used herein to denote any plane or
curved surface in the three-dimensional space. The surface may be
part of a body or may be completely separated therefrom, as it is
usually the case with a field or a pupil plane.
[0056] The term "field plane" is used herein to denote the mask
plane or any other plane that is optically conjugate to the mask
plane.
[0057] The term "pupil plane" is a plane in which (at least
approximately) a Fourier relationship is established to a field
plane. Generally marginal rays passing through different points in
the mask plane intersect in a pupil plane, and chief rays intersect
the optical axis. As usual in the art, the term "pupil plane" is
also used if it is in fact not a plane in the mathematical sense,
but is slightly curved so that, in the strict sense, it should be
referred to as pupil surface.
[0058] The term "uniform" is used herein to denote a property that
does not depend on the position.
[0059] The term "optical raster element" is used herein to denote
any optical element, for example a lens, a prism or a diffractive
optical element, which is arranged, together with other identical
or similar optical raster elements so that each optical raster
element is associated with one of a plurality of adjacent optical
channels.
[0060] The term "optical integrator" is used herein to denote an
optical system that increases the product NAa, wherein NA is the
numerical aperture and a is the illuminated field area.
[0061] The term "condenser" is used herein to denote an optical
element or an optical system that establishes (at least
approximately) a Fourier relationship between two planes, for
example a field plane and a pupil plane.
[0062] The term "conjugated plane" is used herein to denote planes
between which an imaging relationship is established. More
information relating to the concept of conjugate planes are
described in an essay E. Delano entitled: "First-order Design and
the y, y Diagram", Applied Optics, 1963, vol. 2, no. 12, pages
1251-1256.
[0063] The term "field dependence" is used herein to denote any
functional dependence of a physical quantity from the position in a
field plane.
[0064] The term "spatial irradiance distribution" is used herein to
denote how the total irradiance varies over a real or imaginary
surface on which light impinges. Usually the spatial irradiance
distribution can be described by a function I.sub.s(x, y), with x,
y being spatial coordinates of a point on the surface.
[0065] The term "angular irradiance distribution" is used herein to
denote how the irradiance of a light bundle varies depending on the
angles of the light rays that constitute the light bundle. Usually
the angular irradiance distribution can be described by a function
I.sub.a(.alpha., .beta.), with .alpha., .beta. being angular
coordinates describing the directions of the light rays. If the
angular irradiance distribution has a field dependence, I.sub.a
will be also a function of field coordinates, i.e.
I.sub.a=I.sub.a(.alpha.,.beta.,x,y). The field dependence of the
angular irradiance distribution may be described by a set of
expansion coefficients a.sub.ij of a Taylor (or another suitable)
expansion of I.sub.a(.alpha.,.beta.,x,y) in x, y.
BRIEF DESCRIPTION OF THE DRAWINGS
[0066] Various features and advantages of the present disclosure
may be more readily understood with reference to the following
detailed description taken in conjunction with the accompanying
drawings in which:
[0067] FIG. 1 is a schematic perspective view of a projection
exposure apparatus in accordance with one embodiment of the present
disclosure;
[0068] FIG. 2 is an enlarged perspective view of the mask to be
projected by the projection exposure apparatus shown in FIG. 1,
illustrating various deficiencies of the angular irradiance
distribution;
[0069] FIG. 3 is a meridional section through an illumination
system being part of the apparatus shown in FIG. 1;
[0070] FIG. 4 is a perspective view of a first mirror array
contained in the illumination system shown in FIG. 3;
[0071] FIG. 5 is a perspective view of a second mirror array
contained in the illumination system shown in FIG. 3;
[0072] FIG. 6 is a perspective view of an optical integrator
contained in the illumination system shown in FIG. 3;
[0073] FIG. 7 is a schematic meridional section through the first
and the second mirror array shown in FIGS. 4 and 5;
[0074] FIG. 8 is a perspective view on the second mirror array
shown in FIG. 5, but illuminated with two poles;
[0075] FIG. 9 is a perspective view of the optical integrator shown
in FIG. 6, but illuminated with two poles;
[0076] FIG. 10 is a schematic meridional section through a portion
of the illumination system in which only a mirror array, a
condenser and an array of optical raster elements are shown;
[0077] FIGS. 11a and 11b are top views on the second mirror array
and the optical integrator shown in FIG. 3;
[0078] FIG. 12 illustrates an irradiance distribution on a light
entrance facet of the optical integrator;
[0079] FIG. 13 is a graph showing the scan integrated irradiance
distribution along the X direction produced by the light entrance
facet shown in FIG. 12;
[0080] FIG. 14 illustrates another irradiance distribution on a
light entrance facet of the optical integrator;
[0081] FIG. 15 is a graph showing the scan integrated irradiance
distribution along the X direction produced by the light entrance
facet shown in FIG. 14;
[0082] FIG. 16 is a top view on the second mirror array on which a
plurality of light spots produce an irradiance distribution;
[0083] FIG. 17 shows the second mirror array of FIG. 16, but with
several of the micromirrors in an "off"-state;
[0084] FIG. 18 is a top view on the irradiance distribution on a
single light entrance facet for an alternative embodiment;
[0085] FIG. 19 is a graph showing the scan integrated irradiance
distribution along the X direction produced by the light entrance
facet shown in FIG. 18;
[0086] FIGS. 20a to 20c illustrate images of micromirrors on a
light entrance facet and the corresponding irradiance distribution
on the mask;
[0087] FIG. 21 is a graph showing the total irradiance distribution
that is obtained by superimposing the irradiance distributions
shown in FIGS. 20a to 20c;
[0088] FIG. 22 is a schematic meridional section through an
objective, which is contained in the illumination system shown in
FIG. 3, and an additional scattering plate;
[0089] FIG. 23 is a schematic perspective view on an object area on
the second mirror array, an anamorphotic objective and an optical
raster element of the optical integrator;
[0090] FIG. 24 is a schematic meridional section showing the second
mirror array, the objective and a light entrance facet;
[0091] FIG. 25 shows a similar arrangement as in FIG. 24, but with
an off-axis arrangement of the micromirrors and the light entrance
facets;
[0092] FIG. 26 is a meridional section through an embodiment in
which groups of object areas are separated by a gap that is not
imaged on the light entrance facets;
[0093] FIG. 27 is a top view on a second mirror array according to
another embodiment in which the second mirror array includes
passive portions;
[0094] FIG. 28 is a meridional section through an illumination
system according to another embodiment in which a diffractive
optical element is used to define the irradiance distribution on an
LCD panel used as spatial light modulator;
[0095] FIG. 29 is a flow diagram that illustrates important method
steps.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
I. General Construction of Projection Exposure Apparatus
[0096] FIG. 1 is a perspective and highly simplified view of a
projection exposure apparatus 10 in accordance with the present
disclosure. The apparatus 10 includes a light source 11 that may be
realized as an excimer laser, for example. The light source 11 in
this embodiment produces projection light having a center
wavelength of 193 nm. Other wavelengths, for example 257 nm or 248
nm, are envisaged as well.
[0097] The apparatus 10 further includes an illumination system 12
which conditions the projection light provided by the light source
11 in a manner that will be explained below in further detail. The
projection light emerging from the illumination system 12
illuminates an illumination field 14 on a mask 16. The mask 16
contains a pattern 18 formed by a plurality of small features 19
that are schematically indicated in FIG. 1 as thin lines. In this
embodiment the illumination field 14 has the shape of a rectangle.
However, other shapes of the illuminated field 14, for example a
ring segment, are also contemplated.
[0098] A projection objective 20 including lenses L1 to L6 images
the pattern 18 within the illumination field 14 onto a light
sensitive layer 22, for example a photoresist, which is supported
by a substrate 24. The substrate 24, which may be formed by a
silicon wafer, is arranged on a wafer stage (not shown) such that a
top surface of the light sensitive layer 22 is precisely located in
an image plane of the projection objective 20. The mask 16 is
positioned via a mask stage (not shown) in an object plane of the
projection objective 20. Since the latter has a magnification
.beta. with |.beta.|<1, a minified image 18' of the pattern 18
within the illumination field 14 is projected onto the light
sensitive layer 22.
[0099] During the projection the mask 16 and the substrate 24 move
along a scan direction which corresponds to the Y direction
indicated in FIG. 1. The illumination field 14 then scans over the
mask 16 so that patterned areas larger than the illumination field
14 can be continuously imaged. The ratio between the velocities of
the substrate 24 and the mask 16 is equal to the magnification
.beta. of the projection objective 20. If the projection objective
20 does not invert the image (.beta.>0), the mask 16 and the
substrate 24 move along the same direction, as this is indicated in
FIG. 1 by arrows A1 and A2. However, the present disclosure may
also be used in stepper tools in which the mask 16 and the
substrate 24 do not move during projection of the mask.
II. Field Dependent Angular Irradiance Distribution
[0100] FIG. 2 is an enlarged perspective view of the mask 16
containing another exemplary pattern 18. For the sake of simplicity
it is assumed that the pattern 18 includes only features 19 that
extend along the Y direction. It is further assumed that the
features 19 extending along the Y direction are best imaged on the
light sensitive layer 22 with an X dipole illumination setting.
[0101] In FIG. 2 an exit pupil 26a associated with a light bundle
is illustrated by a circle. The light bundle converges towards a
field point that is located at a certain X position of the
illumination field 14 at a first time during a scan cycle. In the
exit pupil 26a two poles 27a, which are spaced apart along the X
direction, represent directions from which projection light
propagates towards this field point. The light energies
concentrated in each pole 27a are assumed to be equal. Thus the
projection light impinging from the +X direction has the same
energy as the projection light impinging from the -X direction.
Since the features 19 are assumed to be uniformly distributed over
the pattern 18, this X dipole illumination setting should be
produced at each field point on the mask 16.
[0102] Another exit pupil denoted by 26b is associated with a light
bundle that converges towards a field point that is located at
another X position of the illumination field 14 at a later time of
the scan cycle. The light energies concentrated in each pole 27b
are again equal. However, the light associated with the poles 27b
are tilted compared to the light cones of light that are associated
with the ideal pole 27a. This means that the field point receives
the same amount of projection light, but the directions from which
the projection light impinges on the field point are not ideal for
imaging the features 19 on the light sensitive layer 22.
[0103] A further exit pupil denoted by 26c is associated with a
point in the illuminated field 14 that is located at still another
X position. Here it is assumed that the directions from which the
projection light impinges on the field point are again ideal for
imaging the features 19. Therefore also the light cones associated
with the poles 27c have the same cone angle and orientation as the
cones associated with the ideal exit pupil 26a. However, the poles
27c are not balanced, i.e. the light energy concentrated in the
poles 27c differs from one another. Thus the projection light
impinging from the +X direction has less energy than the projection
light impinging from the -X direction.
[0104] From the foregoing it becomes clear that the ideal angular
irradiance distribution represented by the exit pupil 26a is not
obtained at each X position in the illumination field 14. The
angular irradiance distribution is therefore field-dependent, i.e.
at different field points the angular irradiance distribution is
different.
[0105] A field dependence may not only occur along the X direction,
but also along the Y direction within the illumination field 14.
Then one point on the mask 16 experiences different angular
irradiance distributions while it passes through the illumination
field 14 during a scan cycle. If a field dependence along the Y
direction (i.e. the scan direction) occurs, it has to be taken into
account that the total effect for a particular field point is
obtained by integrating the different angular irradiance
distributions.
[0106] There is a wide variety of further field-dependent
deviations of a real angular irradiance distribution from the ideal
one. For example, the poles in the exit pupil associated with some
field points may be deformed, blurred or may not have a desired
non-uniform irradiance distribution.
[0107] If field dependent deviations from the ideal angular
irradiance distribution occur, this generally has a negative impact
on the quality of the pattern image that is formed on the light
sensitive layer 22. In particular, the dimensions of the structures
that are produced with the help of the apparatus 10 may vary
inadvertently, and this may compromise the function of the devices
containing these structures. Therefore it is generally desired to
eliminate any field dependence of the illumination setting in the
illumination field 14.
[0108] Sometimes, however, it is desirable to deliberately
introduce a field dependence of the angular irradiance
distribution. This may be expedient, for example, if the projection
objective 20 or the mask 16 have field depending properties that
affect the image of the pattern 18 on the light sensitive layer 22.
Variations of the imaging properties of the projection objective 20
may occur as a result of manufacturing tolerances, aging phenomena
or non-uniform temperature distributions, for example. A field
dependence of the mask 16 often occurs as a result of features that
have different orientations or dimensions, for example. Often field
dependent adverse effects can be successfully reduced by
selectively introducing a field dependence of the angular
irradiance distribution. Since some of these effects change very
rapidly, it is sometimes desired to change the field dependence of
the angular irradiance distribution during a single scan cycle.
III. General Construction of Illumination System
[0109] FIG. 3 is a meridional section through the illumination
system 12 shown in FIG. 1. For the sake of clarity, the
illustration of FIG. 3 is considerably simplified and not to scale.
This particularly implies that different optical units are
represented by one or very few optical elements only. In reality,
these units may include significantly more lenses and other optical
elements.
[0110] In the embodiment shown, the projection light emitted by the
light source 11 enters a beam expansion unit 32 which outputs an
expanded and almost collimated light beam 34. To this end the beam
expansion unit 32 may include several lenses or may be realized as
a mirror arrangement, for example.
[0111] The projection light beam 34 then enters a pupil forming
unit 36 that is used to produce variable spatial irradiance
distributions in a subsequent plane. To this end the pupil forming
unit 36 includes a first mirror array 38 of very small mirrors 40
that can be tilted individually about two orthogonal axes with the
help of actuators. FIG. 4 is a perspective view of the first mirror
array 38 illustrating how two parallel light beams 42, 44 are
reflected into different directions depending on the tilting angles
of the mirrors 40 on which the light beams 42, 44 impinge. In FIGS.
3 and 4 the first mirror array 38 includes only 6.times.6 mirrors
40; in reality the first mirror array 38 may include several
hundreds or even several thousands mirrors 40.
[0112] The pupil forming unit 36 further includes a prism 46 having
a first plane surface 48a and a second plane surface 48b that are
both inclined with respect to an optical axis OA of the
illumination system 12. At these inclined surfaces 48a, 48b
impinging light is reflected by total internal reflection. The
first surface 48a reflects the impinging light towards the mirrors
40 of the first mirror array 38, and the second surface 48b directs
the light reflected from the mirrors 40 towards an exit surface 49
of the prism 46. The angular irradiance distribution of the light
emerging from the exit surface 49 can thus be varied by
individually tilting the mirrors 40 of the first mirror array 38.
More details with regard to the pupil forming unit 36 can be
gleaned from US 2009/0116093 A1.
[0113] The angular irradiance distribution produced by the pupil
forming unit 36 is transformed into a spatial irradiance
distribution with the help of a first condenser 50. The condenser
50, which may be dispensed with in other embodiments, directs the
impinging light towards a digital spatial light modulator 52 that
is configured to reflect impinging light in a spatially resolved
manner. To this end the digital spatial light modulator 52 includes
a second mirror array 54 of micromirrors 56 that are arranged in a
mirror plane 57 and can be seen best in the enlarged cut-out C of
FIG. 3 and the enlarged cut-out C' of FIG. 5. In contrast to the
mirrors 40 of the first mirror array 38, however, each micromirror
56 of the second mirror array 54 has only two stable operating
states, namely an "on" state, in which it directs impinging light
via a first objective 58 towards an optical integrator 60, and an
"off" state, in which it directs impinging towards a light
absorbing surface 62.
[0114] The second mirror array 54 may be realized as a digital
mirror device (DMD), as they are commonly used in beamers, for
example. Such devices may include up to several million
micromirrors that can be switched between the two operating states
many thousands times per second.
[0115] Similar to the pupil forming unit 36, the spatial light
modulator 52 further includes a prism 64 having an entrance surface
65 that is arranged perpendicular to the optical axis OA and a
first plane surface 66a and a second plane surface 66b that are
both inclined with respect to the optical axis OA of the
illumination system 12. At these inclined surfaces 66a, 66b
impinging light is reflected by total internal reflection. The
first surface 66a reflects the impinging light towards the
micromirrors 56 of the second mirror array 54, and the second
surface 66b directs the light reflected from the micromirrors 56
towards a surface 68 of the prism 64.
[0116] If all micromirrors 56 of the second mirror array 54 are in
their "on" state, the second mirror array 54 has substantially the
effect of a plane beam folding mirror. However, if one or more
micromirrors 56 are switched to their "off" state, the spatial
irradiance distribution of the light emerging from the mirror plane
57 is modified. This can be used, in a manner that will be
explained further below in more detail, to produce a field
dependent modification of the angular light distribution on the
mask 16.
[0117] As it already has been mentioned above, the light emerging
from the prism 64 passes through the first objective 58 and
impinges on the optical integrator 60. Since the light passing
through the first objective 58 is almost collimated, the first
objective 58 may have a very low numerical aperture (for example
0.01 or even below) and thus can be realized with a few small
spherical lenses. The first objective 58 images the mirror plane 57
of the spatial light modulator 52 onto the optical integrator
60.
[0118] The optical integrator 60 includes, in the embodiment shown,
a first array 70 and a second array 72 of optical raster elements
74. FIG. 6 is a perspective view of the two arrays 70, 72. Each
array 70, 72 includes, on each side of a support plate, a parallel
array of cylinder lenses extending along the X and the Y direction,
respectively. The volumes where two cylinder lenses cross form
optical raster elements 74. Thus each optical raster element 74 may
be regarded as a microlens having cylindrically curved surfaces.
The use of cylinder lenses is advantageous particularly in those
cases in which the refractive power of the optical raster elements
74 shall be different along the X and the Y direction. A different
refractive power is involved if the square irradiance distribution
on the optical integrator 60 shall be transformed into a
slit-shaped illumination field 14, as this is usually the case. The
surface of the optical raster elements 74 pointing towards the
spatial light modulator 52 will be referred to in the following as
light entrance facet 75.
[0119] The optical raster elements 74 of the first and second array
70, 72 respectively, are arranged one behind the other in such a
way that one optical raster element 74 of the first array 70 is
associated in a one to one correspondence with one optical raster
element 74 of the second array 72. The two optical raster elements
74, which are associated with each other, are aligned along a
common axis and define an optical channel. Within the optical
integrator 60 a light beam which propagates in one optical channel
does not cross or superimpose with light beams propagating in other
optical channels. Thus the optical channels associated with the
optical raster elements 74 are optically isolated from each
other.
[0120] In this embodiment a pupil plane 76 of the illumination
system 12 is located behind the second array 72; however, it may
equally be arranged in front of it. A second condenser 78
establishes a Fourier relationship between the pupil plane 76 and a
field stop plane 80 in which an adjustable field stop 82 is
arranged.
[0121] The field stop plane 80 is optically conjugated to a raster
field plane 84 which is located within or in close proximity to the
light entrance facets 75 of the optical integrator 60. This means
that each light entrance facet 75 in the raster field plane 84 is
imaged onto the entire field stop plane 80 by the associated
optical raster element 74 of the second array 72 and the second
condenser 78. The images of the irradiance distribution on the
light entrance facet 75 within all optical channels superimpose in
the field stop plane 80, which results in its very uniform
illumination of the mask 16. Another way of describing the uniform
illumination of the mask 16 is based on the irradiance distribution
which is produced by each optical channel in the pupil plane 76.
This irradiance distribution is often referred to as secondary
light source. All secondary light sources commonly illuminate the
field stop plane 80 with projection light from different
directions. If a secondary light source is "dark", no light
impinges on the mask 16 from a (small) range of directions that is
associated with this particular light source. Thus it is possible
to set the desired angular light distribution on the mask 16 by
simply switching on and off the secondary light sources formed in
the pupil plane 76. This is accomplished by changing the irradiance
distribution on the optical integrator 60 with the help of the
pupil forming unit 36.
[0122] The field stop plane 80 is imaged by a second objective 86
onto a mask plane 88 in which the mask 16 is arranged with the help
of a mask stage (not shown). The adjustable field stop 82 is also
imaged on the mask plane 88 and defines at least the short lateral
sides of the illumination field 14 extending along the scan
direction Y.
[0123] The pupil forming unit 36 and the spatial light modulator 52
are connected to a control unit 90 which is, in turn, connected to
an overall system control 92 illustrated as a personal computer.
The control unit 90 is configured to control the mirrors 40 of the
pupil forming unit 36 and the micromirrors 56 of the spatial light
modulator 52 in such a manner that the angular irradiance
distribution in the mask plane 88 is uniform, or a desired field
dependence angular irradiance distribution is obtained.
[0124] In the following it will be described how this is
accomplished.
IV. Function and Control of the Illumination System
1. Pupil Forming
[0125] FIG. 7 schematically illustrates how the pupil forming unit
36 produces an irradiance distribution on the micromirrors 56 of
the spatial light modulator 52. For the sake of simplicity the
prisms 46, 64 are not shown.
[0126] Each mirror 40 of the first mirror array 38 is configured to
illuminate a spot 94 on the mirror plane 57 of the spatial light
modulator 52 at a position that is variable by changing a
deflection angle produced by the respective mirror 40. Thus the
spots 94 can be freely moved over the mirror plane 57 by tilting
the mirrors 40 around their tilt axes. In this way it is possible
to produce a wide variety of different irradiance distributions on
the mirror plane 57. The spots 94 may also partly or completely
overlap, as this is shown at 95. Then also graded irradiance
distributions may be produced.
[0127] FIG. 8 is a perspective view, similar to FIG. 5, on the
second mirror array 54 contained in the spatial light modulator 52.
Here it is assumed that the pupil forming unit 36 has produced an
irradiance distribution on the second mirror array 54 that consists
of two square poles 27 each extending exactly over 6.times.6
micromirrors 56. The poles 27 are arranged point-symmetrically
along the X direction.
[0128] The objective 58 forms an image of this irradiance
distribution on the light entrance facets 75 of the optical
integrator 60, as this is shown in FIG. 9. Here it is assumed that
all micromirrors 56 are in the "on"-state so that the irradiance
distribution formed on the second mirror array 54 is identically
reproduced (apart from a possible scaling due to a magnification of
the objective 58) on the light entrance facets 75 of the optical
integrator 60. For the sake of simplicity images of gaps that
separate adjacent micromirrors 56 of the second mirror array 54 are
disregarded. The regular grid shown on the light entrance facets 75
represent an image of the borderlines of the micromirrors 56, but
this image does not appear outside the poles 27 and is shown only
in FIG. 9 for illustrative reasons.
2. Field Dependence
[0129] Since the light entrance facets 75 are located in the raster
field plane 84, the irradiance distribution on the light entrance
facets 75 is imaged, via the optical raster elements 74 of the
second array 72 and the second condenser 78, on the field stop
plane 80.
[0130] This will now be explained with reference to FIG. 10 which
is an enlarged and not to scale cut-out from FIG. 3. Here only two
pairs of optical raster elements 74 of the optical integrator 60,
the second condenser 78 and the intermediate field stop plane 80
are shown schematically.
[0131] Two optical raster elements 74 that are associated with a
single optical channel are referred to in the following as first
microlens 101 and second microlens 102. The microlenses 101, 102
are sometimes referred to as field and pupil honeycomb lenses. Each
pair of microlenses 101, 102 associated with a particular optical
channel produces a secondary light source 106 in the pupil plane
76. In the upper half of FIG. 10 it is assumed that converging
light bundles L1a, L2a and L3a illustrated with solid, dotted and
broken lines, respectively, impinge on different points of the
light entrance facet 75 of the first microlens 101. After having
passed the two microlenses 101, 102 and the condenser 78, each
light bundle L1a, L2a and L3a converges to a focal point F1, F2 and
F3, respectively. From the upper half of FIG. 10 it becomes clear
that points, where light rays impinge on the light entrance facet
75, and points where these light rays pass the field stop plane 80
(or any other conjugated field plane), are optically conjugate.
[0132] The lower half of FIG. 10 illustrates the case when
collimated light bundles L1b, L2b and L3b impinge on different
regions of the light entrance facet 75 of the first microlens 101.
This is the more realistic case because the light impinging on the
optical integrator 60 is usually substantially collimated. The
light bundles L1b, L2b and L3b are focused in a common focal point
F located in the second microlens 102 and then pass, now collimated
again, the field stop plane 80. Again it can be seen that, as a
result of the optical conjugation, the region where a light bundle
L1b, L2b and L3b impinges on the light entrance facet 75
corresponds to the region which is illuminated in the field stop
plane 80. As a matter of course, these considerations apply
separately for the X and the Y direction if the microlenses 101,
102 have refractive power both along the X and Y direction.
[0133] Therefore each point on a light entrance facet 75 directly
corresponds to a conjugated point in the intermediate field stop
plane 80 (and hence in the illumination field 14 on the mask 16).
If it is possible to selectively influence the irradiance on a
point on a light entrance facet 75, it is thus possible to
influence the irradiance of a light ray that impinges on the
conjugated point in the illumination field 14 from a direction that
depends on the position of the light entrance facet 75 with respect
to the optical axis OA of the illumination system. The larger the
distance between the light entrance facet 75 from the optical axis
OA is, the larger is the angle under which the light ray impinges
on the point on the mask 16.
3. Modifying Irradiance on Light Entrance Facets
[0134] In the illumination system 12 the spatial light modulator 52
is used to modify the irradiance on points on the light entrance
facets 75. In FIG. 9 it can be seen that each pole 27 extends over
a plurality of small areas that are images of the micromirrors 56.
If a micromirror is brought into an "off" state, the conjugated
area on the light entrance facet 75 will not be illuminated, and
consequently no projection light will impinge on a conjugated area
on the mask from the (small) range of directions that is associated
with this particular light entrance facet 75.
[0135] This will be explained in more detail with reference to
FIGS. 11a and 11b which are top views on the micromirrors 56 of the
spatial light modulator 52 and on the light entrance facets 75 of
the optical integrator 60, respectively.
[0136] The thick dotted lines on the second mirror array 54 divide
its mirror plane 57 into a plurality of object areas 110 each
including 3.times.3 micromirrors 56. The objective 58 forms an
image of each object area 110 on the optical integrator 60. This
image will be referred to in the following as image area 110'. Each
image area 110' completely coincides with a light entrance facet
75, i.e. the image areas 110' have the same shape, size and
orientation as the light entrance facets 75 and are completely
superimposed on the latter. Since each object area 110 includes
3.times.3 micromirrors 56, the image areas 110' also include
3.times.3 images 56' of micromirrors 56.
[0137] In FIG. 11a there are eight object areas 110 that are
completely illuminated by the pupil forming unit 36 with projection
light. These eight object areas 110 form the two poles 27. It can
be seen that in some of the object areas 110 one, two or more
micromirrors 56d represented as black squares have been controlled
by the control unit 90 such that they are in an "off"-state in
which impinging projection light is not directed towards the
objective 58, but towards the absorber 62. By switching
micromirrors between the "on" and the "off" state it is thus
possible to variably prevent projection light from impinging on
corresponding regions within the image areas 110' on the light
entrance facets 75, as this is shown in FIG. 11b. These regions
will be referred to in the following as dark spots 56d'.
[0138] As has been explained above with reference to FIG. 10, the
irradiance distribution on the light entrance facets 75 is imaged
on the field stop plane 80. If a light entrance facet 75 contains
one or more dark spots 56d', as this is illustrated in the upper
portion of FIG. 12, the irradiance distribution produced in the
mask plane 88 by the associated optical channel will have dark
spots at certain X positions, too. If a point on a mask passes
through the illumination field 14, the total scan integrated
irradiance will thus depend on the X position of the point in the
illuminated field 14, as this is shown in the graph of FIG. 13.
Points in the middle of the illumination field 14 will experience
the highest scan integrated irradiance, because they do not pass
through dark spots, and points at the longitudinal ends of the
illumination field 14 will receive total irradiances that are
reduced to different extents. Thus the field dependence of the
angular light distribution on the mask 16 can be modified by
selectively bringing one or more micromirrors 56 of the spatial
light modulator 52 from an "on"-state into the "off"-state.
[0139] In a foregoing it has to be assumed that each object area
110, which is imaged on one of the light entrance facets 75,
contains only 3.times.3 micromirrors 56. Thus the resolution along
the cross-scan direction X that can be used to modify the field
dependence of the angular light distribution is relatively coarse.
If the number of micromirrors 56 within each object area 110 is
increased, this resolution can be improved.
[0140] FIG. 14 illustrates a top view on one of the light entrance
facets 75 for an embodiment in which 20.times.20 micromirrors 56
are contained in each object area 110. Then more complicated scan
integrated irradiance distributions along the X direction can be
achieved on the mask 16, as this is illustrated in the graph shown
in FIG. 15.
4. Clipping
[0141] In the foregoing it has been assumed that the pupil forming
unit 36 illuminates poles 27 on the second mirror array 54 that
exactly extend over four adjacent object areas 110. Generally,
however, it will be difficult to produce such an irradiance
distribution with sharp edges.
[0142] The spatial light modulator 52 may also be used to clip a
blurred irradiance distribution in the mirror plane 57 by bringing
those micromirrors 56 into the "off"-state that lie outside the
object areas 110 that shall be illuminated.
[0143] This is illustrated in FIGS. 16 and 17 in which an
irradiance distribution 96 on the second mirror array 54 are shown.
Here it is assumed that the movable light spots 94 produced by the
mirrors 40 of the pupil forming unit 36 are superimposed to form
four poles. If all micromirrors 56 of the spatial light modulator
52 are in the "on" state as shown in FIG. 16, the blurred
irradiance distribution 96 would be imaged on the light entrance
facets 75. If those micromirrors 56 surrounding the desired object
areas 110 are brought into the "off"-state as shown in FIG. 17,
they form a frame that delimits the poles and thus produces sharp
edges of the intensity distribution on the light entrance
facets.
5. Relative Rotation
[0144] In the embodiments described so far it has been assumed that
the micromirrors 56 are aligned parallel to the borderlines of the
object areas 110. The rectangular grid formed by the micromirrors
56 is then parallel to the rectangular grid which is formed by the
light entrance facets 75. This results in irradiance distributions
as shown in FIGS. 13 and 15 in which the irradiance along one
"column" of micromirrors 56 is always uniform. Thus only stepped
irradiance distributions can be produced on the light entrance
facets 75.
[0145] Sometimes it is desirable to produce irradiance
distributions that are not stepped, but contain inclined portions.
This can be achieved if the two rectangular grids are not arranged
parallel to each other, but with an angle .alpha., as this is shown
in FIG. 18. Here the images 56' of the micromirrors 56 form a grid
114 which forms an angle .alpha. with the lateral sides of the
light entrance facet 75. Then the centers of adjacent micromirrors
56 are aligned along a straight line having an image 116 that forms
the same angle .alpha. to a boundary line of the light entrance
facet 75. If this angle .alpha. is distinct from m45.degree. with
m=0, 1, 2, 3, . . . , the irradiance distribution will not have the
shape of stepped profiles as shown in FIGS. 13 and 15.
[0146] FIG. 19 is a graph that illustrates the scan integrated
irradiance distribution along the X direction for the rotated
arrangement shown in FIG. 18. Some particular X positions are
indicated in FIG. 18 with broken lines. If the angle .alpha. is
distinct from m45.degree. with m=0, 1, 2, 3, . . . , the
degeneration is reduced so that a desired attenuation can be
obtained at more different X positions. In other words, it is thus
possible to effectively increase the resolution along the X
direction that is available to modify the field dependence of the
angular irradiance distribution.
6. Gaps--Lateral Displacement
[0147] As mentioned further above, it is usually inevitable that
small gaps are formed between adjacent micromirrors 56 of the
second mirror array 38. Images of these gaps are formed on the
light entrance facets 75 and also on the mask 16. If these images
extend parallel to the cross-scan direction X, this is of little
concern because of the integrating effect that results from the
scan operation. However, dark lines extending parallel to the scan
direction Y could not be compensated by the integrating effect.
[0148] FIG. 20a shows in the upper portion a top view on one of the
light entrance facets 75 in which the images of the gaps are
denoted by 118'. The graph in the lower portion of FIG. 20a
illustrates the irradiance distribution along the cross-scan
direction X that is produced by this particular light entrance
facet 75 in the mask plane 88. If all light entrance facets 75
would produce dark lines 120 at the same X positions, no projection
light would reach points on the mask 16 at these positions.
[0149] FIGS. 20b and 20c show other light entrance facets 75 in
which the gap images 118' are laterally displaced along the
cross-scan direction X to different degrees. Consequently also the
dark lines 120 in the irradiance distributions shown in the lower
portion of these figures are laterally displaced. Since the
irradiance distributions produced by each optical channel are
superimposed in the mask plane 88, the dark lines 120 are averaged
out, as this is shown in FIG. 21. The larger the number of light
entrance facets 75 is and the smaller the dark lines 120 are, the
more approximates the irradiance distribution I(x) in the mask
plane 88 a uniform distribution.
6. Gaps--Scattering Plate
[0150] Alternatively or additionally, a scattering plate 122 may be
arranged in an optical path between the optical light modulator 52
and the mask plane 88 in order to avoid dark lines on the mask
plane 88 caused by gap images 118'. Suitable positions of the
scattering plate 122 are between the optical light modulator 52 and
the objective 58, between the objective 58 and the optical
integrator 60, or in the vicinity of the field stop plane 80.
[0151] FIG. 22 is a schematic meridional section showing several
micromirrors 56 of the spatial light modulator 52, the objective 58
and the scattering plate 122 arranged in between. A gap 118 between
two adjacent micromirrors 56 is assumed to have a width d, and the
axial distance between the scattering plate 122 and the light exit
surface 57 of the spatial light modulator 52 is denoted by b. If
the characteristic scattering angle .beta. of the scattering plate
122 is approximately d/b, the image of the gap 118 formed on the
light entrance facet 75 is sufficiently blurred. If the scattering
angle .beta. is significantly larger than d/b, the desired spatial
resolution for the field dependence of the irradiance and the
angular irradiance distribution is reduced. If the scattering angle
.beta. is too small, the images of the gaps will still be prominent
on the light entrance facets 75.
7. Rectangular Object Areas
[0152] In the embodiments described above it has been assumed that
the number of micromirrors 56 along the scan direction Y and the
cross-scan direction X is identical. Then a rectangular grid of
square micromirrors 56 perfectly fits into a square light entrance
facet 75 of the optical integrator 60.
[0153] The number N.sub.X of micromirrors 56 along the cross-scan
direction X determines the resolution that is available for
adjusting the field dependence of the irradiance and the angular
irradiance distribution. This number should be as high as
possible.
[0154] The number N.sub.Y of micromirrors 56 along the scan
direction Y may be significantly smaller because of the integrating
effect caused by the scan operation. Illustratively speaking, a
plurality of optical channels adjacent along the scan direction Y
may contribute to the reduction of the irradiance on a point on the
mask 16 during a scan cycle. This does not apply to optical
channels that are adjacent along the cross-scan direction X.
[0155] This suggests that the object area 110 may well be
rectangular, with the length along the cross-scan direction X being
larger (for example two times and preferably at least five times
larger) than the length of the object area along the scan direction
Y. Assuming micromirrors 56 having equal dimensions along the
directions X and Y, this implies that the number N.sub.X of
micromirrors 56 along the cross-scan direction X is larger than the
number N.sub.Y along the scan direction Y.
[0156] If a rectangular object area 110 shall be imaged on a square
light entrance facet 75, the objective 58 has to be anamorphotic.
More specifically, the absolute value of the magnification M has to
be smaller along the cross-scan direction X than along the scan
direction Y, i.e. |M.sub.X|<|M.sub.Y|. This is illustrated in
FIG. 23 in which two cylinder lenses 124, 126 of the objective 58
are arranged between a single rectangular object area 110 and the
light entrance facet 75 of an optical raster element 74. If the
length of the object area 110 along the cross-scan direction X is
Lx and the length along the scan direction Y is L.sub.Y,
|M.sub.X/M.sub.Y| should be equal to L.sub.Y/L.sub.X.
[0157] A similar result is achieved if not the objective 58, but
the subsequent condenser 78 is anamorphotic so that its focal
length f is different for the X and Y directions. If the objective
58 is rotational symmetric so that M.sub.X=M.sub.Y, the irradiance
distributions on the light entrance facets 75 will be rectangular
with the same aspect ratio L.sub.X/L.sub.Y as the object area 110.
This rectangular irradiance distribution is then expanded by the
anamorphotic condenser 78 so that a square irradiance distribution
is obtained in the field stop plane 80 and the subsequent mask
plane 88. This approach may involve a redesign of the optical
integrator 60 because the condenser's different focal lengths along
the directions X, Y have to be compensated by the refractive power
of the optical raster elements 74.
8. Arrangement of Mirror Plane
[0158] It is usually preferred if the chief rays of the projection
light impinge perpendicularly on the optical integrator 60. Then
also the mirror plane 57, which is imaged by the objective 58 on
the light entrance facets 75, is arranged perpendicularly to the
optical axis OA, as this is shown in FIG. 24. In such a parallel
arrangement of the micromirrors 56 and the light entrance facets 75
the micromirrors 56 have to produce a deflection angle which is
distinct from zero if they are in the "on"-state. This is different
to conventional digital mirror devices (DMD) in which all mirror
surfaces are arranged in a single plane if they are in the
"on"-state.
[0159] Additionally or alternatively, the second mirror array 54
and the light entrance facets 75 may be arranged in off-axis
regions of the object field and the image field of the objective
58, respectively. As it is shown in FIG. 25, it is then possible to
use an objective 58 such that it is not telecentric on the object
side, but telecentric on the image side. This means that chief rays
forming an angle with the optical axis OA on the object side are
nevertheless parallel to the optical axis OA on the image side.
9. Grouping Object Areas
[0160] If the number of micromirrors 56 in each object area 110 and
also the number of optical channels (and thus of the light entrance
facets 75) shall be large, the total number of micromirrors 56 in
the second mirror array 54 may become huge. Since it might be
difficult to provide a single second mirror array 54 that includes
such a huge number of micromirrors 56, it is envisaged to split up
the second mirror device into several sub-units. More specifically,
the second mirror array 54 may be combined from several groups of
object areas, wherein the groups are separated from each other by
dark areas (i.e. an area from which no projection light emerges)
that are not imaged on the light entrance facets. Each group may be
realized as a single device, for example a digital mirror device
(DMD).
[0161] FIG. 26 is a schematic meridional section through the second
mirror array 54 and the objective 58 according to this embodiment.
It is assumed that the second mirror array 54 includes two groups
54-1, 54-2 each realized as digital mirror device (DMD). Each group
54-1, 54-2 includes three object areas 110 that extend over a
plurality of micromirrors 56. The two groups 54-1, 54-2 are
separated by a dark area 130 which is absorptive and on which no
projection light should be directed by the pupil forming unit
36.
[0162] The objective 58 is configured to combine the images 110' of
the object areas 110 so that they abut at least substantially
seamlessly on the optical integrator 60. There each image area 110'
completely coincides with one of the light entrance facets 75. To
this end the objective 58 produces magnified images of the object
areas 110 in an intermediate image plane 132 with the help of a
first array of lenses 134. The objective 58 further includes an
array of second lenses 136 that is arranged in the intermediate
image plane 132. Common imaging optics 138 then image the
intermediate image plane 134, in which the magnified images of the
groups already abut, on the light entrance facets 75 of the optical
integrator 60. In this way the dark areas 130 between the groups
54-1, 54-2 is not imaged by the objective 58 on the optical
integrator 60.
10. Active and Passive Areas
[0163] Instead of providing a huge number of micromirrors 56 so
that the light irradiance distribution on every light entrance
facet 75 can be modified, it may be envisaged to modify the light
irradiance distribution not on all, but only on some light entrance
facets 75.
[0164] This approach is illustrated in FIG. 27 which is a top view
on the second mirror 54. Groups 54-1 to 54-6 including at least one
and preferably several object areas 110 each including a plurality
of micromirrors 56 are arranged within the mirror plane 57. Also in
this embodiment each group may be realized as a digital mirror
device (DMD). Since object areas 110 are referred to in the
following as active object areas.
[0165] The entire area outside the groups 54-1 to 54-6 is
configured as a plane reflective surface 140 which is provided with
openings 142 in which the groups 54-1 to 54-6 are received. The
reflective surface 140 may be considered as being formed as a
combination of passive object areas that are also imaged on the
light entrance facets 75, but in which no spatial light modulation
is possible. The entire second mirror array 54 thus has the effect
of a plane mirror in which the deflection angle of certain portions
(namely the portions where the groups 54-1 to 54-6 are arranged)
can be individually controlled.
[0166] This approach exploits the fact that it is usually not
necessary to modify the irradiance distribution on every light
entrance facet 75 for corrective purposes. Correction of the pole
balance in the case of a dipole illumination setting, for example,
involves only that the irradiance in one pole is reduced; the
irradiance distribution in the other pole may remain as it is. For
that reason the groups 54-1 to 54-6 are arranged
point-symmetrically with respect to the optical axis OA. For any
arbitrary arrangement of poles it is then possible to reduce the
irradiance in a field dependent manner using the active object
areas 110 contained in the group that is illuminated by one of the
poles.
[0167] In this context it may be expedient to concentrate more
projection light in the pole that illuminates a group with the
active object areas. This pole is then used to perform the desired
field dependent corrections of the irradiance and/or the angular
irradiance distribution. The (albeit small) light loss which is
inevitably involved in such a correction compensates the originally
unbalanced illumination of the poles.
[0168] In FIG. 27 this is illustrated for two poles 27a, 27b. The
pole 27a which one is located in the third group 54-3 is brighter
than the other pole 27b in the passive area 140.
11. Diffractive Optical Element and LCD
[0169] FIG. 28 is a meridional section similar to FIG. 3 of an
alternative embodiment of an illumination system 12. In this
illumination system the pupil forming unit 52 is replaced by a
diffractive optical element 142, zoom optics 144 and a pair of
axicon elements 146, 148.
[0170] The spatial light modulator 52 in this embodiment is formed
by an LCD panel including a two dimensional array of minute LCD
cells whose optical activity can be controlled individually by the
control unit 90. If the projection light produced by the light
source 11 is not sufficiently polarized, an additional polarizer
may be inserted in the light path in front of the spatial light
modulator 52.
[0171] As a matter of course, the embodiments shown in FIGS. 3 and
28 can also be combined in different ways so that, for example, a
diffractive optical element 142 is used together with the second
mirror array 54 as spatial light modulator 52.
V. Important Method Steps
[0172] Important method steps of the present disclosure will now be
summarized with reference to the flow diagram shown in FIG. 29.
[0173] In a first step S1 an object area on a spatial light
modulator is completely illuminated.
[0174] In a second step S2 the object area is imaged on a light
entrance facet of an optical integrator.
[0175] In a third step S3 it is prevented that all light associated
with a point in the object area impinges on the light entrance
facet.
* * * * *